1. Field of the Invention
The present invention relates to a planar heat generating element having a resistance heat generator for generating heat upon a passage of electric current therethrough, a fixing device including the planar heat generating element, and an image forming apparatus including the fixing device.
2. Description of the Related Art
As a fixing device for use in an electrophotographic image forming apparatus such as a copying machine and a printer, a fixing device of heat-roller fixing type has been in wide use. The fixing device of heat-roller fixing type includes a pair of rollers, a fixing roller and a pressure roller, that are brought into contact with each other under pressure. By means of a heating section composed for example of a halogen lamp, which is placed in each of or one of the pair of rollers interiorly thereof, the pair of rollers are heated to a predetermined fixing temperature. With the pair of rollers kept in a heated state, such as a recording paper sheet, which is a recording medium having formed thereon an unfixed toner image, is fed to a region where the pair of rollers make pressure-contact with each other, namely a fixing nip region. Upon the recording paper sheet passing through a pressure contact portion, the toner image is fixed into place under application of heat and pressure.
Incidentally, a fixing device for use in a color image forming apparatus generally employs an elastic roller constructed by forming an elastic layer made for example of silicone rubber on a surface layer of the fixing roller. By designing the fixing roller as an elastic roller, it is possible for the surface of the fixing roller to become elastically deformed so as to conform to irregularities of the unfixed toner image, wherefore the fixing roller makes contact with the toner image so as to cover the surface of the toner image. This makes it possible to perform satisfactory thermal fixation on the unfixed color toner image that is larger in toner adherent amount than a monochromatic toner image. Moreover, by virtue of a deflection-releasing effect exerted by the elastic layer in the fixing nip region, it is possible to provide enhanced releasability for a color toner that is more susceptible to occurrence of offset than a monochromatic toner. Further, since the fixing nip region is convexly curved in a radially-outward direction so as to define a so-called reverse nip configuration, it is possible to attain higher paper-stripping capability. That is, a paper stripping action can be produced without using a stripping portion such as a stripping pawl (self-stripping action), wherefore image imperfection caused by the provision of the stripping portion can be eliminated.
Incidentally, in such a fixing device provided in a color image forming apparatus, it is necessary to make a nip width of a fixing nip section wide in order to correspond to increase in speed. One available method of increasing the fixing nip width is to increase the thickness of the elastic layer of the fixing roller and the diameter of the fixing roller. However, in a fixing roller having an elastic layer, the elastic layer can not sufficiently conduct heat, thus, in a case where a heating section is provided inside the fixing roller, there is a problem that a temperature of the fixing roller is not followed when a process speed is increased. On the other hand, when a diameter of the fixing roller is increased, there is a problem that it takes longer time to warm up or power consumption is increased.
As a fixing device provided in a color image forming apparatus to solve such problems, Japanese Unexamined Patent Publication JP-A 10-307496 (1998) discloses a fixing device of belt fixing type that is configured so that a fixing belt is supported around a fixing roller and a heating roller and the fixing roller and a pressure roller are brought into pressure-contact with each other with the fixing belt interposed therebetween. In the fixing device of belt fixing type, since the fixing belt with small heat capacity is heated, it takes short time to warm up and it is not necessary to incorporate a heat source such as a halogen lamp in the fixing roller, thus making it possible to provide a thick elastic layer with low hardness made of sponge rubber and the like and to secure a wide nip width.
Furthermore, JP-A 2002-333788 discloses a fixing device of planar heat generating belt fixing type with a heating section as a planar heat generating element. In the fixing device of planar heat generating belt fixing type, when heat capacity of the heating section is reduced, the planar heat generating element as the heating section directly generates heat at the same time, thus a thermal response speed is also enhanced compared to a system in which a heating roller is heated indirectly using a halogen lamp or the like and it is possible to attain further shortening of a time for warm up and more energy saving.
However, in a fixing system using a resistance heat generating element as the planar heat generating element, a member with small heat capacity is used as a substrate so that a surface temperature is determined by a balance between transmitted heat and radiated heat, thus heat radiation volume from both ends of the roller is increased when heat is generated. Accordingly, the temperature of the both ends of the planar heat generating element is lower than that of its center part and it is difficult to obtain uniform temperature distribution over the all areas in a width direction. As a result, when such a fixing device is applied to an image forming apparatus such as a copier and a printer, variance is generated in a toner fixing temperature and the printing quality is deteriorated.
Japanese Unexamined Patent Publication JP-A 2006-215056 discloses a fixing device devised to solve such a problem. FIG. 9 is a view showing the structure of a planar heat generating element 200 provided in the fixing device according to the related art. The planar heat generating element 200 has a heat generating pattern 201 formed of a heat generating line 202 that constitutes a certain plane as a whole. The heat generating pattern 201 includes a plurality of linear portions 202a which are arranged side by side substantially in parallel with one another, of which each is inclined at a predetermined angle θ with respect to an axial direction X of a fixing belt. The heat generating pattern 201 is formed on a surface of an insulating layer 203. The heat generating line 202 has its both ends connected via a control section 205 to a power source 204. In this structure, energization control is exercised on the basis of temperatures detected by a non-illustrated temperature sensor.
In the planar heat generating element 200 thus constructed, formation of the plurality of heat generating patterns 201 makes it possible to obtain a plurality of heat generating regions. By actuating the power source 204 so as to apply a voltage to the plurality of heat generating regions in a selective manner, it is possible to render the distribution of temperatures over the areas of boundaries among the heat generating regions more even, and thereby heat the fixing belt to a desired temperature with a smoothed temperature distribution.
The planar heat generating element 200 is, at the insulating layer 203, fixedly disposed on the back surface of a substrate disposed in contact with the fixing belt. However, the heat generating line 202 includes a bend portion 202b. When the heat generating line 202 having the bend portion 202b receives application of a voltage from the power source 204 and thereby electric current of high magnitude is passed therethrough, then the electric current flows intensively through the bend portion 202b of the heat generating line 202 interiorly thereof. Therefore, the bend portion 202b liberates heat excessively, with the result that that part of the heat generating line 202 which constitutes the bend portion 202b or that part of the insulating layer 203 which faces the bend portion 202b may be caused to peel off from the substrate. In consequence, since the heat generated from the peeled heat generating line 202 cannot be transmitted to the heat generating substrate, it becomes impossible to apply heat to the fixing belt in such a manner as to render the temperature distribution on the surface thereof uniform. Furthermore, the excessive heat generation from the bend portion 202b leads to the possibility that the heat generating line 202 suffers from a fracture or the insulating layer 203 catches fire.
Particularly, in a case where the heat generating line 202 and the insulating layer 203, as well as the insulating layer 203 and the substrate, are not kept in intimate contact with each other uniformly and sufficiently, or in a case where the planar heat generating element 200 needs to be disposed securely on a substrate having a curved surface, a stress is concentrated excessively on the bend portion 202b. As a result, the heat generating line 202 is apt to peel due to the resultant excessive heat generation.